A typical prior art semiconductor optical power receiver, or photocell, includes a pn junction formed between a base layer, which may typically be n-type, and an emitter layer, which may typically be p-type. The base layer is formed on an intrinsic substrate with a substantially thick buffer layer isolating the base layer from the substrate. The prior art optical power receiver further includes an optically transparent window layer on the emitter layer and a semiconductor cap layer formed on the window. A metallic grid to make electrical connection to the device is formed on the cap layer. The cap layer is then etched to expose the window layer except under the metallization of the grid. The cap layer provides isolation of the metallization from the pn junction with respect to self-doping of the semiconductor material from the metallization. Additionally, the cap layer provides for electrical conduction between the emitter and metallization.
When the optical energy is incident upon the emitter layer, the absorbed photons generate electrons and holes in the semiconductor material of the base and emitter layers. The number of carriers generated is dependent upon the wavelength of the radiation, thereby establishing the spectral response of the device. This spectral response is a function of the doping concentration of the base and emitter layers and also of their thicknesses.
The minority carriers generated in the base by the optical energy are collected across the pn junction. Once across the junction, such carriers become majority carriers and flow as an electrical current to the nearest grid line in the metallic grid. A disadvantage and limitation of the prior art device is the characteristic emitter sheet resistance of the emitter layer which causes a resistive loss associated with the passage of the current through the emitter to the grid. The grid lines of the metal grid need therefore be optimized to minimize the resistive loss without causing obscuration of the emitter layer to incident optical energy. For example, in optimizing the grid pattern, as the grid lines are spaced more closely together, the resistive losses are minimized. However, the optical obscuration increases until there is a complete loss of absorbed optical energy.
To further reduce the resistive losses, the sheet resistance of the emitter layer can also be reduced by increasing either the thickness of the emitter layer or its doping concentration. However, these parameters of the emitter layer affect the spectral response of the device as hereinabove discussed supra. Therefore, the enhancement of spectral response may increase emitter sheet resistance and, conversely, the minimization of emitter sheet resistance will degrade the spectral response according to the immediately hereinabove described techniques of the prior art. Accordingly, a primary disadvantage and limitation of prior art semiconductor optical power receivers is that their optimal design combines both optical and electrical efficiency.